“See with what entire freedom the whaleman takes his handful of lamps—often but old bottles and vials, though—to the copper cooler at the try-works, and replenishes them there, as mugs of ale at a vat. He burns, too, the purest of oil, in its unmanufactured, and, therefore, unvitiated state; a fluid unknown to solar, lunar, or astral contrivances ashore. It is sweet as early grass butter in April. He goes and hunts for his oil, so as to be sure of its freshness and genuineness, even as the traveller on the prairie hunts up his own supper of game.”
‘Early grass butter in April’ simply means the butter made from the milk of cows (or other animals) fed on grass growing in the early spring, which is said to be especially delicate and pure tasting. The butter made from this milk is therefore said to be especially tasty, or ‘sweet’ (which doesn’t necessarily mean sweet as honey is sweet).
‘Solar, lunar or astral contrivances ashore’ I take to be a whimsical way of saying that the oil produced on land can never be as fine as that produced at sea, however complex or sophisticated the land-based machinery made to make the oil is.
